In well known, commercial, boiling water nuclear power reactors, a pressure vessel contains a core of fuel material submerged in a liquid such as light water, which serves both as a working fluid and a neutron moderator.
The water is circulated through the core, whereby a portion thereof is converted to steam. The steam is taken from the pressure vessel and applied to a prime mover, such as a turbine, for driving an electric generator. The turbine exhaust steam is condensed and, along with any necessary makeup water, is returned to the pressure vessel by a condensate delivery system.
Typically, nuclear reactors are provided with water level control systems which monitor water level within the vessel, steam outflow from the vessel, and feedwater inflow into the vessel. Water level control systems manipulate the operation of the condensate delivery system to control water level in the reactor vessel. Should steam outflow exceed feedwater inflow, the water level control system will tend to direct an increase in feedwater flow into the vessel. Similarly, for an excess of feedwater flow over steam flow, the fluid level control system will tend to direct a decrease in feedwater flow into the vessel. An indication of water level imbalance in the vessel will, however, dominate a signal generated by a steam and feedwater flow imbalance. A high water level indication will result in a demand for a reduction in feedwater flow. A low water level indication will result in a demand for an increase in feedwater flow. U.S. Pat. No. 4,302,288 discloses exemplary reactor water level control systems and is expressly incorporated herein by reference.
Feedwater pumps in condensate delivery systems are typically driven by one of two means. Where feedwater pumps are driven by electric motors, feedwater flow can be controlled by directing the feedwater through a flow control valve and positioning the valve, according to the demands of the water level control system, to reduce or increase resistance to flow. In some nuclear plants, feedwater pumps are driven by turbines which utilize steam from the reactor vessel. In such cases, feedwater flow can be controlled by varying the amount of steam delivered to these turbines. A flow control valve is included in the steam delivery pipes to permit such control.
Adjustments affecting feedwater flow through the feedwater pump also affect water pressure at both the pump outlet and inlet. By way of example, opening a valve used for flow control in the feedwater line will result in an increase in flow with a commensurate increase in the load on the motor driving the pump. Pressure at the pump inlet will fall. As another example, an increasing quantity of steam delivered to a turbine driving a pump will cause the pump to accelerate with an attendant decrease in inlet pressure. Feedwater flow will increase.
The typical condensate delivery system comprises a plurality of centrifugal pumps. The feedwater pumps are those pumps which raise feedwater water pressure to the level of pressure inside the reactor vessel. The feedwater is typically at an elevated temperature. Water pressure is subject to variation at various internal points of a centrifugal pump during pump operation. Although average water pressure increases as the water penetrates the pump, local pressure within the pump may, through turbulence and other factors, drop considerably below pump inlet pressure. Should local pressure fall enough, flash boiling of the water with consequent pump cavitation can result. This adversely effects pump efficiency and can result in damage to the pump.
Boiling occurs at saturation of the water at local pressure. That is to say, water is saturated when further additions of heat, or a decrease in local pressure, causes some of the water to change to a vapor. If a sufficient difference between the enthalpy of the water in the pump inlet and the enthalpy at saturation of the water at local pressure within the pump is maintained, boiling is prevented. This difference in enthalpy from inlet to pump interior is termed subcooling and is expressed in units of enthalpy, e.g. BTU/LBM. The subcooling required by any given pump varies with water temperature. Such characteristics of centrifugal pumps have long been known and data thereon is generally available from the pump manufacturer. Heretofore, protective measures to prevent pump cavitation have typically employed a pressure trigger to shut down the pump prime mover whenever pump inlet pressure has fallen below a predetermined value. Such pressure triggers operate at the chosen predetermined value for all water temperatures. Pressure trigger protective measures have been utilized in nuclear power plants.
While the required subcooling for a given pump may increase or decrease for various combinations of temperature and pressure, adequate subcooling for a given pump can be obtained at lower pump inlet pressures as water temperature falls. Consequently, unnecessary triggering of protective steps can occur where a simple pressure trigger is used. In a nuclear power plant, a pressure-triggered feedwater pump shutdown resulting in a partial cut-off of water flow to the reactor could undesirably necessitate a scram of the reactor. Such pump system shutdowns are more likely to occur when maintaining maximum feedwater flow to the vessel is especially important to avoid a reactor scram. An example of such a case would be when reactor water level is low, and the feedwater level control system is attempting to increase feedwater flow.
Another concern with existing systems is that increased flow demand results not only in reduced pressure, but in increased load on the pump motor, where motors are used. As the motor slows with increased load from its normal operating speed, it consumes more power and draws more current. For especially high load demands, the excessive current drawn can trigger a relay which shuts off the motor, again potentially resulting in a reactor scram.
The operating history of nuclear reactors shows that cavitation and pump motor overloading in pump systems occurs far more frequently in feedwater pump systems than in condensate pump systems. Thus various embodiments of the invention are depicted as employed with feedwater pumps.
Accordingly, it is an object of the present invention to provide a system for controlling the feedwater flow rate, which overrides demands for feedwater flow that are not sustainable by the condensate delivery system.
It is another object of the present invention to monitor the subcooling of a liquid before introduction of the liquid into a motive pump and to compare the subcooling to the subcooling required in the liquid to prevent cavitation in the pump.
It is a still further object of the present invention to monitor a parameter indicative of power consumed by a pump prime mover and to effect changes in pump load to reduce power consumption by the prime mover when power consumption is excessive.
It is an object of the present invention to allow the condensate delivery system to achieve maximum feedwater flow under adverse system operating conditions.
It is an additional object of the present invention to monitor system parameters most directly indicative of conditions within a liquid flow line and actuate protective apparatus on the basis thereof.
It is a yet further object of the present invention to prevent cascading shutdowns of equipment resulting in unnecessary scrams of a nuclear reactor.